Influence of heat treatment parameters on the microstructure and properties of some zinc-based alloys

Abstract

The present investigation deals with microstructure and property related changes in a zinc-based alloy induced by varying solutionizing and ageing durations and (solutionizing) temperatures. The influence of partially replacing copper by nickel and silicon in the alloy composition has also been studied on similar lines. The microstructure of the as cast, nickel/silicon-free alloy revealed primary α dendrites surrounded by eutectoid and eutectic α+η in the interdendritic regions along with the metastable ε phase. The addition of nickel and silicon partially altered the basic microstructure of the alloy by forming primary silicon particles and intermetallic compounds. Solutionizing led to the breaking of the dendritic structure and redistribution of the alloying elements whilst ageing formed the T′ phase. The morphology of silicon particles and the (nickel containing) intermetallic compound was noted to be unaltered in the nickel and silicon containing alloy during the heat treatment. The hardness of the alloys increased during solutionizing whilst it reduced after ageing when compared with that of the as cast sample value. The presence of silicon and nickel led to an increase in the hardness of the alloy. It also enabled the alloy to retain a higher hardness during the heat treatment. An increased solutionizing duration led to an initial increase in the hardness with a peak value being obtained which was followed by a reduction in the hardness. The presence of nickel and silicon in the alloy reduced the tendency of a reduction in hardness beyond the peak hardness. The microconstituents also lowered the detrimental influence of the coarsening of the T′ phase. The density of the nickel/silicon-free alloy varied in a narrow range. However within this range, solutionizing caused an increase in the density with duration followed by a reduction and finally a steady state value. The addition of silicon and nickel did not affect the density of the alloy to any great extent. The trend followed by the electrical resistivity was identical to that of hardness. Furthermore, the presence of silicon and nickel increased the resistivity of the alloy. Changes in the properties of the alloys during the heat treatment have been explained on the basis of microstructural alterations caused by the heat treatment.